When collecting Japanese netsuke most of the true antique netsuke, as against lots of modern copies, will normally have two connecting cord holes (known as himotoshi), but one will be much larger than the other!
The netsuke was a handling piece, that was on a cord to a compartment (Sagemono), often this was an inro (a case of several compartments), that was kept closed by a corded bead (ojime) between the inro and the netsuke. The kimono had no pockets so the inro was worn hanging from the kimono sash (obi); the netsuke was then, pushed up under the sash, thus trapping and holding the Sagemono in place.
The reason for the larger hole in the netsuke was that once the cord had been threaded through the inro (or Sagemono) and ojime (the netsuke would then have been threaded on last) it was possible to completely hide the one and only cord knot inside the larger hole.
So the way the smaller hole connected to the larger hole was very well formed providing a smooth curved link that was easy to thread the cord through. Often the larger hole was hollowed out even more, under the surface, in order to provide plenty of space for the knot.
The Manju, often used instead of a netsuke, also had plenty of space to hide the knot within the opening of the two sections.
In my personal view, although there are some very fine netsuke that relied on being threaded through a tail, or a leg, instead of having the usual himotoshi, there was then quite a disadvantage in use, as there was no longer anyway of concealing the cord knot.
Another important factor is the position of such himotoshi, they were always placed so that, as the cord hangs down, the netsuke carving is presented attractively and the right way up. But, these cord holes were also placed in such a way that they did not detract from any of the detailed carving.
Many of the poor quality later netsuke (these were never used, but were really made simply to cash in, on the increasing value of netsuke) are often easy to spot. When inspecting a netsuke if the two holes are of equal size and poorly formed (often these are just two drilled holes angled to meet each other) then you can be sure this is not a good netsuke!
Sadly one can no longer just rely on judging the himotoshi to identify a modern copy, as some now do have smaller and larger holes, especially the recent resin copies of netsuke.